The clinically useful .beta.-lactam antibiotics include a wide variety of compounds: cephalosporins such as cephalexin, cefaclor, and cefamandol; totally synthetic "oxa"-cephalosporins such as moxalactam; and a variety of monocyclic .beta.-lactams as well as bicyclic .beta.-lactams. Because the .beta.-lactam antibiotics represent such an important therapeutic class, considerable effort is directed toward more efficient methods for their preparation.